Writes the value to the
BTChan or blocks if the channel is full.
c2 = setMaxOfBTChan c1 mx Using the same underlying
set a new maximum number of messages,
mx. If the current size
is greater than
mx then no messages are dropped, but writes
will block till the size goes lower than
c1 concurrently is possible, but
c2 writes will block at the new
maximum while writes to
c1 will block at the new, making it biased
against whichever writer has the channel with the smaller bound.